The number of indications for ultrasound examination during pregnancy is rising. In addition, experts have expanded the gestational age range for which fetal ultrasound assessment is appropriate to include the first trimester. Considering the increased use of ultrasound, should clinicians heighten their concern regarding the safety of this technique? Are there ways to minimize potential risks to the fetus? What do clinicians need to know about the safety of this important imaging modality during pregnancy? The answers are not as straightforward as one might think.
This article reviews the potential impact of ultrasound on the fetus and ways through which the sonographer can minimize risk and maximize safety. A key element of this is understanding how to use the output display standard (ODS) during all obstetric ultrasound examinations. The ODS, which is displayed on screen during an ultrasound examination, provides an approximation of biologic risk generated during the exam. It is currently the safety mechanism in place for all ultrasound examinations, and the American College of Obstetricians and Gynecologists (ACOG) supports its use.1
Ultrasound differs from other imaging modalities in that it is nonionizing and, thus, considered safer for fetal evaluation.2 Nevertheless, obstetric ultrasound may produce biologic changes through 1 of 2 mechanisms: thermal or nonthermal.
Thermal bioeffects refer to biologic changes associated with a rise in temperature in the targeted tissue.3 Although some ultrasound waves reflect off the exposed tissue and are used for the generation of images, others are absorbed by the tissue and the energy is converted into heat.3 This is important given the potential teratogenicity of fetal exposure to significant temperature elevations.4
Nonthermal bioeffects, frequently referred to as mechanical bioeffects, are biologic changes resulting from ultrasound insonation without a pathophysiologic rise in temperature. Although this concept is somewhat abstract, it nonetheless appears to be relevant in fetal ultrasound safety.5 Examples of mechanical bioeffects observed with diagnostic ultrasound include cavitation, acoustic streaming, radiation forces, and free-radical generation. The term cavitation, which is frequently encountered when investigating mechanical bioeffects, refers to the interaction between an ultrasound wave and a gas bubble.5 Ultrasound waves can cause gas bubbles to move, expand, or collapse. At the cellular level, even slight movement of gas bubbles can potentially disrupt cellular connections.
The ODS was created by the American Institute of Ultrasound in Medicine and the National Electrical Manufacturers Association to approximate the risks for thermal and mechanical bioeffects encountered during an ultrasound examination. As noted earlier, use of the ODS is supported by ACOG1 as well as by the US Food and Drug Administration (FDA).6 The ODS includes the thermal index (TI) and the mechanical index (MI), which are calculated by the ultrasound machine and displayed in a corner on screen if the machine is capable of generating a TI or MI greater than 1. The higher the TI or MI value, the greater the probability of a thermal or mechanical bioeffect.6 As a general rule, an index value less than 1 is considered safe (Table 1).6 Displaying the index number on the screen allows the clinician or sonographer to monitor the number while scanning. If the index value is greater than 1, the sonographer can make necessary adjustments. Minimizing the exposure time, changing the ultrasound mode, or adjusting such settings as the power output all can affect the TI or MI values (Table 2).6,7
The FDA is responsible for designating an upper limit for the allowable output of ultrasound machines. In 1976, this value was set fairly low, at 94 mW/cm2 , for fetal exposure.6 As technology improved, clearer images and Doppler capability required more powerful output settings. To permit clinical use of these newer ultrasound machines, the FDA increased the allowable output for obstetric ultrasound machines in 1992 to 720 mW/cm2 : nearly 8 times higher than the previous limit.6 Currently, instead of ultrasound safety being regulated strictly by the FDA, the responsibility rests largely with the sonographer, who is required to ensure that the limits of the ODS are not exceeded during the ultrasound examination.6
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